The present invention relates to a technology of nonlinearity compensation in optical communication networks.
Three basic physical factors, that are known as limiting the achievable bit-rate in optical communication links, are chromatic dispersion, power losses and non-linearity effect.
Nonlinearity, that is a nonlinear phase shift accumulated by a light signal while being transmitted via an optic fiber, is generated by the Kerr effect in silica glass. Owing to that effect, the refraction coefficient of a material changes with the intensity of the optical signal, according to the following formula:
n=n0+K|E|2,xe2x80x83xe2x80x83(1)
where K is the Kerr coefficient.
It is known to those skilled in the art, that power losses can be compensated by all-optical Erbium-doped or Raman amplifiers periodically installed into a long fiber link.
Dispersion can also be compensated by periodically inserting relatively short elements with the opposite sign and large absolute value of the dispersion, which makes it possible to have the average dispersion practically equal to zero. Among such elements is, for example, a fiber of a different kind, or very short pieces of a fiber with the Bragg grating written on it.
Unlike these linear effects, direct compensation of accumulated nonlinear phase shift seems physically impossible nowadays, since conventional optical materials are all self-focussing i.e., they have the Kerr effect of the positive sign only.
On the other hand, known in the art are the so-called nonlinear crystals capable of producing higher harmonics of an optical signal from its fundamental harmonic. Such crystals, for example potassium titanyl phosphate (KTP), potassium dihydrogen phosphate (KDP), barium borate optical crystals (BBO) and the like are used in various types of laser generators. Examples of such systems can be found in JP 08201862 A2, U.S. Pat. No. 6,047,011, and others.
It is the objective of the invention to provide a method, a device and a system for compensating the nonlinearity in fiber optic links, such as communication links.
According to the first aspect of the invention, the above objective can be achieved by providing:
a method for compensating nonlinearity accumulating in an optical signal passing via an optical fiber link, the nonlinearity being created due to the positive Kerr effect of optical fibers forming the link, the method comprising:
passing the optical signal at a particular wavelength via the fiber link,
conveying said optical signal via one or more compensating devices capable of creating an artificial negative Kerr effect for the said particular wavelength.
In the method, each of said compensating devices is based upon at least one element selected from the following non-exhaustive list including: a second-harmonic-generating (SHG) optical crystal, a second-harmonic-generating (SHG) polymer fiber (for example, obtained by poling), and a semiconductor waveguide.
Preferably, the step of conveying the optical signal via said one or more compensating devices comprises conveying the optical signal along a multi-segment trajectory in at least one of the compensating devices, thereby arranging an extended optical path for said optical signal. For example, the optical path can be extended by forming a multi-path xe2x80x9czig-zagxe2x80x9d trajectory of the incoming optical beam in the compensating device.
The method is most efficient for gradual compensation of the nonlinearity in the fiber optic link, wherein the step of conveying the optical signal comprises passing it via a chain of more than one said compensating devices spanned by sections of said optical fiber link. In other word, if more than one compensating devices are inserted in the link and spaced from one another, each of them will compensate nonlinearity accumulated in a preceding section of the fiber link.
For a case of multi-channel transmission, the method further includes steps of: passing via the said optical fiber link one or more additional optical signals with respective wavelengths different from one another and from said particular wavelength, and
conveying said at least one additional optical signal via said one or more compensating devices,
wherein said one or more compensating devices being capable of creating the negative Kerr effect with respect to the wavelengths of the one or more additional optical signals.
The above-described method can be used for WDM transmission format.
If compensation of the nonlinearity is nonuniform for different optical channels in the multi-channel transmission, optical channels with better compensation of nonlinearity can be used for transmitting information having higher priority.
According to a second aspect of the invention, there is proposed a suitable system for compensating nonlinearity appearing in an optical fiber link due to the positive Kerr effect, the system comprising
one or more optical fibers forming a part of the link, and creating said positive Kerr effect,
one or more compensating devices being inserted in said link and capable of creating the negative Kerr effect for at least one optical channel.
In accordance with the further aspect of the invention, there is also provided a compensating device for compensation of nonlinearity appearing in optical fiber links due to the positive Kerr effect created by optical fibers, said compensating device being capable of creating the negative Kerr effect for one or more optical wavelengths.
The proposed compensating device comprises at least one element from the following non-exhaustive list including a second-harmonic-generating optical crystal, a second-harmonic-generating polymer fiber (such as a poled polymeric fiber), and a semiconductor waveguide.
In the preferred embodiment of the invention, the compensating device comprises the second-harmonic-generating (SHG) optical crystal selected from a non-exhaustive list comprising KTP, KDP and BBO.
According to one specific implementation, the crystal has a cubic form and is covered at its two opposite facets by mirror surfaces (for internal reflection), leaving two windows at said opposite facets for an incoming optical beam and an outgoing optical beam respectively, intended for the fundamental harmonic of the beam to arrange between said two windows an extended optical path of the optical beam through the crystal.
Actually, the crystal may have other shapes, be covered by mirror surfaces not only at its opposite sides, and may have more than two optical ports for incoming and outgoing beams, thus enabling selection of any pair of such ports for a specific length of the trajectory.
Preferably, the length of the optical path in the crystal is selected so as to ensure the minimal output power of the second harmonic generated in the crystal.
The compensating device is preferably integrated with an optical amplifier and placed immediately after said amplifier.
In practice, the compensating device may form part of an optical network node.
According to yet another aspect of the invention, there is also provided a method for producing a compensating device for nonlinearity compensation in an optical fiber, the method comprising: obtaining a second-harmonic-generating (SHG) optical crystal and ensuring that the sign of the Kerr effect created by said crystal for at least one optical wavelength is negative.
Preferably, the above method comprises a step of controlling the sign and value of the Kerr effect to be created in the crystal by periodic poling of said SHG optical crystal.
Also, the method preferably includes selecting (by suitable calculation) of a length of the optical path in the crystal to ensure the minimum output power of the second harmonic generated in the crystal.
Further aspects and details of the invention will become apparent from the following description.